1. Field of the Invention
This invention relates to a silicone rubber composition and more particularly, to a silicone rubber composition of the hot air vulcanization type which, when vulcanized, gives no problems of void deficiency, surface tackiness and sanitation and is useful for wire coating as having good heat resistance and transparency.
2. Description of the Prior Art
Silicone rubbers have wide utility in various fields because of their good heat and cold resistances and good electric characteristics. A variety of techniques of processing such rubbers have been practised depending on the manner of application.
In particular, silicone rubbers have been vulcanized in different ways which depend on the type of rubber and the physical properties required for final cured product. Generally speaking, heat treatment in the presence of organic peroxides is the most popular for this purpose. A number of the organic peroxides may be mentioned including benzoyl peroxide, p-chlorobenzoyl peroxide, 2,4-chlorobenzoyl peroxide, dicumyl peroxide, di-t-butyl peroxide, 2,5-dimethyl-2,5-di(t-butyl peroxy)hexane, t-butyl perbenzoate, t-butylcumyl peroxide and the like. For obtaining moldings of good quality, such as tubings, electric wires, sheets and the like, by continuous hot air vulcanization by extrusion, 2,4-dichlorobenzoyl peroxide is only one vulcanizing agent which is now available for this purpose.
However, 2,4-dichlorobenzoyl peroxide has problems such as a corrosion problem on metal and a safety and hygienic problem of decomposed side products. Moreover, the cured products obtained from silicone rubbers using the peroxide are not necessarily satisfactory in heat resistance and compression set under sealed conditions. Organic peroxides for hot air vulcanization which are designed to solve the above problems have been proposed in U.S. Pat. No. 4,061,704 and include, for example, t-alkyl peroxyalkyl carbonates such as t-butyl peroxyoctadecyl carbonate, t-amyl peroxy-2,6,8-trimethyl-4-nonyl carbonate, t-amyl peroxyoctadecyl carbonate and the like. However, the use of the t-alkylperoxyalkyl carbonates alone does not provide cured product which can stand practical use. Additionally, such carbonates leave a kind of impurity having a large molecular weight in the product. The removal of the impurity is not easy. If it remains in the silicone rubber product, the heat resistance becomes poor.
On the other hand, it is known that for hot air vulcanization of silicone rubbers containing unsaturated groups, they are subjected to addition reaction in the presence of an organohydrogenpolysiloxane and a platinum catalyst. However, this silicone rubber composition is shorter in pot life than those compositions using organic peroxides, leading to a tendency toward scorching and limiting its application only to a certain field. Thus, such a composition will not be of the general-purpose type.
Aside from the organic peroxides indicated before, there are known other organic peroxides which are applicable for hot air vulcanization and can solve the problems on blooming and sanitation involved by the impurity or residue formed by decomposition. Examples of such organic peroxides include bis(ortho-methyl)benzoyl peroxide (Japanese Laid-open Patent Application No. 59-18758), bis(dimethyl-substituted benzoyl) peroxide (Japanese Laid-open Patent Application No. 60-16968), bis(trimethyl-substituted benzoyl) peroxide (Japanese Laid-open Patent Application No. 60-163860), bis(ortho-methoxybenzoyl)peroxide (Japanese Laid-open Patent Application No. 59-232145) and the like. However, cured products containing these peroxides undesirably exhibit surface tackiness which is greater than that produced by known the 2,4-dichlorobenzoyl peroxide. This surface tackiness is unfavorable as a value of final product. When these peroxides are used for medical service, it is necessary that in a UV absorption spectrum test, the absorbance at a wavelength of 220 m.mu. be not larger than 0.3. To attain this, postcuring should be inconveniently carried out to a satisfactory extent.